Liquid crystal device

ABSTRACT

A liquid crystal device is constituted by a pair of substrates and a ferroelectric liquid crystal disposed between the substrates. At least one of the pair of substrates has thereon an alignment film of a polyimide having a recurring unit of the following formula (I): ##STR1## wherein R 1  denotes a tetravalent organic residue; and X 1  -X 8  independently denote an alkyl group having 1-15 carbon atoms, an alkoxy group having 1-15 carbon atoms, CF 3  group, halogen or hydrogen atom with the proviso that at least one of X 1  -X 4  is not hydrogen atom and at least one of X 5  -X 8  is not hydrogen atom.

FIELD OF THE INVENTION AND RELATED ART

This invention relates to a liquid crystal device to be used in a liquidcrystal display device or a liquid crystal-optical shutter, etc.,particularly a liquid crystal device using a ferroelectric liquidcrystal, more particularly to a liquid crystal device improved indisplay characteristics through improvement in initial alignment of theliquid crystal molecules.

A display device of the type which controls transmission of light incombination with a polarizing device by utilizing the refractive indexanisotropy of ferroelectric liquid crystal molecules has been proposedby Clark and Lagerwall (Japanese Laid-Open Patent Application No.107216/1981, U.S. Pat. No. 4,367,924). The ferroelectric liquid crystalhas generally chiral smectic C phase (SmC*) or H phase (SmH*) of anon-helical structure and, in the SmC* or SmH* phase, shows a propertyof assuming either one of a first optically stable state and a secondoptically stable state responding to an electrical field applied theretoand maintaining such a state in the absence of an electrical field,namely bistability, and also has a quick responsiveness to the change inelectrical field. Thus, it is expected to be utilized in a high speedand memory type display device and particularly to provide a large-area,high-resolution display in view of its excellent function.

For an optical modulating device using a ferroelectric liquid crystalhaving such bistability to exhibit desirable driving characteristics, itis required that the liquid crystal disposed between a pair ofsubstrates should be in such a molecular alignment state that conversionbetween the above two stable states may occur effectively irrespectiveof the application of an electrical field.

Further, in a liquid crystal device utilizing birefringence of a liquidcrystal, the transmittance under right angle cross nicols is given bythe following equation:

    I/IO=sin.sup.2 4θ·sin.sup.2 (Δnd/λ)π,

wherein

I₀ : incident light intensity,

I: transmitted light intensity,

θ: tilt angle,

Δn: refractive index anisotropy,

d: thickness of the liquid crystal layer,

λ: wavelength of the incident light.

The tilt angle θ in the above-mentioned non-helical structure isrecognized as a half of an angle between the average molecular axisdirections of liquid crystal molecules in a twisted alignment in a firstorientation state and a second orientation state. According to the aboveequation, it is shown that a tilt angle θ of 22.5 degrees provides amaximum transmittance and the tilt angle θ in a non-helical structurefor realizing bistability should desirably be as close as possible to22.5 degrees in order to provide a high transmittance and a highcontrast.

A method for aligning a ferroelectric liquid crystal should desirably besuch that molecular layers each composed of a plurality of molecules ofa smectic liquid crystal are aligned uniaxially along their normals, andit is desirable to accomplish such an alignment state by a rubbingtreatment which requires only a simple production step.

As an alignment method for a ferroelectric liquid crystal, particularlya chiral smectic liquid crystal in a non-helical structure, onedisclosed in U.S. Pat. No. 4,561,726 has been known for example.

However, when a conventional alignment method, particularly one using apolyimide film treated by rubbing, is applied for alignment of aferroelectric liquid crystal in a non-helical structure exhibitingbistability reported by Clark and Lagerwall, the following problems areencountered.

That is, according to our experiments, it has been found that a tileangle θ (an angle shown in FIG. 3 as described below) in a ferroelectricliquid crystal with a non-helical structure obtained by alignment withan alignment control film of the prior art has become smaller ascompared with a tilt angle H (the angle H is a half of the apex angle ofthe cone shown in FIG. 2 as described below) in the ferroelectric liquidcrystal having a helical structure. Particularly, the tilt angle θ in aferroelectric liquid crystal with a non-helical structure obtained byalignment with alignment control films of the prior art was found to begenerally on the order of 3-8 degrees, and the transmittance at thattime was at most about 3 to 5%.

Thus, according to Clark and Lagerwall, the tilt angle in aferroelectric liquid crystal with a non-helical structure realizingbistability should have the same angle as the tilt angle in theferroelectric liquid crystal having a helical structure, but in fact thetilt angle θ in a non-helical structure is smaller than the tilt angle Hin a helical structure. More specifically, it has been found that thetilt angle θ in a non-helical structure becomes smaller than the tiltangle θ because of a twist alignment of liquid crystal molecules in thenon-helical structure. Thus, in a ferroelectric liquid crystal having anon-helical structure, liquid crystal molecules are aligned with a twistfrom a molecular axis adjacent to an upper substrate to a molecular axisadjacent to a lower substrate continuously at a certain twist angle.This leads to a phenomenon that the tilt angle θ in the non-helicalstructure is smaller than the tilt angle H in the helical structure.

Further, in an alignment state of a chiral smectic liquid crystalattained by a conventional polyimide alignment film subjected to arubbing treatment, when a liquid crystal is supplied with a voltage ofone polarity for switching from a first optically stable state (e.g., awhite display state) to a second optically stable state (e.g., a blackdisplay state) and then the voltage of one polarity is removed, theferroelectric liquid crystal layer is supplied with a reverse electricfield Vrev due to the presence of the polyimide film as an insulatinglayer between the electrode and the liquid crystal layer, and thereverse electric field Vrev has caused an after-image during display.The generation of the above-mentioned reverse electric field has beenreported in "Switching characteristic of SSFLC" by Akio Yoshida,"Preprint for Liquid Crystal Forum, October 1987" p.p. 142-143.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide aferroelectric liquid crystal device having solved the above-mentionedproblems, particularly a ferroelectric liquid crystal device whichprovides a large tilt angle of a chiral smectic liquid crystal in anon-helical structure and provides a display capable of displaying ahigh-contrast image and yet free from after-image.

According to the present invention, there is provided a liquid crystaldevice, comprising: a pair of substrates and a ferroelectric liquidcrystal disposed between the substrates; at least one of said pair ofsubstrates having thereon an alignment film comprising a polyimidehaving a recurring unit of the following formula (I): ##STR2## whereinR₁ denotes a tetravalent organic residue; and X₁ -X₈ independentlydenote an alkyl group having 1-15 carbon atoms, an alkoxy group having1-15 carbon atoms, CF₃ group, halogen or hydrogen atom with the provisothat at least one of X₁ -X₄ is not hydrogen atom and at least one of X₅-X₈ is not hydrogen atom.

These and other objects, features and advantages of the presentinvention will become more apparent upon a consideration of thefollowing description of the preferred embodiments of the presentinvention taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view of an embodiment of the liquidcrystal device according to the present invention.

FIG. 2 is a perspective view showing schematically an alignment of achiral smectic liquid crystal having a helical structure.

FIG. 3 is a perspective view showing schematically an alignment state ofa chiral smectic liquid crystal having a non-helical structure.

FIG. 4 is a schematic sectional view showing an alignment state of achiral smectic liquid crystal aligned according to the presentinvention.

FIG. 5 is an illustration of C-director alignments in a uniformalignment state.

FIG. 6 is an illustration of C-director alignments in a splay alignmentstate.

FIGS. 7A and 7B are plan views illustrating tilt angles θ in a uniformalignment state and a splay alignment state, respectively.

FIG. 8 is a sectional view showing a charge distribution, a direction ofa spontaneous polarization P_(S) and a direction of a reverse electricfield Vrev.

FIG. 9 is a schematic plan view illustrating changes in tilt angle θduring and after application of an electric field.

FIGS. 10 and 11 are graphs showing optical response characteristicsaccording to a conventional device and the present invention,respectively.

FIG. 12 is a waveform diagram illustrating driving waveforms used in anembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic sectional view of an embodiment of the liquidcrystal device according to the present invention.

The liquid crystal device comprises a pair of substrates (glass plates)11a and 11b which are coated with transparent electrodes 12a and 12b ofIn₂ O₃, ITO (indium tin oxide), etc., 200-1000 Å-thick insulating films13a and 13b of SiO₂, TiO₂, Ta₂ O₅, etc., and 50-1000 Å-thick alignmentcontrol films 14a and 14b.

In this instance, the alignment control films 14a and 14b have beentreated by rubbing, as a uniaxial aligning treatment, in directionswhich are parallel to each other and in the same direction (indicated byarrows A in FIG. 1). A chiral smectic liquid crystal 15 is disposedbetween the substrates 11a and 11b, and the spacing between thesubstrates 11a and 11b is set to provide the liquid crystal layer 15with a thickness (e.g., 0.1-3 microns) which is sufficiently small tosuppress the formation of a helical structure of the chiral smecticliquid crystal 15 by disposing spacer beads 16 of, e.g., silica,alumina, etc. between the substrates 11a and 11b, whereby the chiralsmectic liquid crystal 15 assumes a bistable alignment state. The thusformed cell structure is sandwiched between a pair of polarizers 17a and17b arranged in cross nicols.

According to our experiments, by using an alignment method using aspecific polyimide alignment film treated by rubbing as explained withreference to Examples described hereinafter, there has been realized analignment state which provides a large optical contrast between a brightand a dark state, particularly with respect to non-selected pixelsduring multiplexing drive as disclosed in U.S. Pat. No 4,655,561, etc.,and also is free from a delay in optical response leading to a problemof after-image in a display at the time of switching during suchmultiplexing drive.

According to the present invention, at least one of the alignment films14a and 14b is constituted by a polyimide having a recurring unit of theformula (I) below, which polyimide has been obtained bydehydrocyclization of a polyamide acid obtained by condensation of thecorresponding tetracarboxylic dianhydride and diamine. ##STR3##

In the above formula (I), the tetravalent organic residue group R₁ isnot particularly restricted but may preferably by an aromaticring-containing group, examples of which may include: ##STR4##

Among these tetravalent organic groups, the following may beparticularly preferred: ##STR5##

Further, when X₁ -X₈ in the diamine component-constituting the recurringunit of the formula (I) are alkyl or alkoxy groups, they should have1-15 carbon atoms, preferably 1-10 carbon atoms, more preferably 1-5carbon atoms. When X₁ -X₈ are halogen atoms, fluorine atoms arepreferred.

The diamine component is not particularly restricted but preferredexamples thereof may include those of the following formulae: ##STR6##

Hereinbelow, specific examples of the polyimide recurring unitrepresented by the general formula (I) will be enumerated: ##STR7##

The polyimide constituting the alignment film of the present inventionmay preferably include a recurring unit of, e.g., the above-mentionedformulae (1)-(58), so as to provide a number-average molecular weight of10⁴ -10⁵, particularly 3×10⁴ -8×10⁴ as measured in its polyamide acidprecursor form by GPC (gel permeation chromatography) corresponding tothe molecular weights of standard polystyrene samples.

In order to form a film 14a or 14b of the polyimide on a substrate, asolution of a polyamide acid as a precursor of the polyimide prepared asdescribed above in a solvent, such as dimethylformamide,dimethylacetoamide, dimethylsulfoxide or N-methylpyrrolidone at aconcentration of 0.01-40 wt. % may be applied onto the substrate byspinner coating, spray coating, roller coating, etc., and heated at100°-350° C., preferably 200°-300° C., to cause dehydro-cyclization. Thethus-formed polyimide film may be rubbed with a cloth, etc.

The polyimide film may be formed in a thickness of, e.g., 30 Å-1 μm,preferably 200-2000 Å, so as to also function as an insulating film. Inthis case, the insulating films 13a and 13b can be omitted. Further, incase of forming the polyimide film on the insulating film 13a or 13b,the polyimide 100 Å or less.

The liquid crystal material 15 used in the present invention maypreferably be one showing a phase transition from isotropic phasethrough cholesteric phase and smectic A phase into chiral smectic Cphase in the course of temperature decrease. Particularly, a chiralsmectic liquid crystal showing a helical pitch of 0.8 micron or longerin cholesteric phase (measured at a mid temperature in the cholestericrange). Preferred examples of such a liquid crystal material may includeliquid crystal materials (1)-(5) below comprising the following liquidcrystals [A], [B] and [C] in the indicated proportions by weight.##STR8##

Liquid Crystal Material

(1) [A]₉₀ /[B]₁₀

(2) [A]₈₀ /[B]₂₀

(3) [A]₇₀ /[B]₃₀

(4) [A]₆₀ /[B]₄₀

(5) [C]

FIG. 2 is a schematic illustration of a ferroelectric liquid crystalcell (device) for explaining operation thereof. Reference numerals 21aand 21b denote substrates (glass plates) on which a transparentelectrode of, e.g., In₂ O₃, SnO₂, ITO (indium-tin-oxide), etc., isdisposed, respectively. A liquid crystal of an SmC*-phase (chiralsmectic C phase) or SmH*-phase (chiral smectic H phase) in which liquidcrystal molecular layers 22 are aligned perpendicular to surfaces of theglass plates is hermetically disposed therebetween. Full lines 23 showliquid crystal molecules. Each liquid crystal molecule 23 has a dipolemoment (P⊥) 24 in a direction perpendicular to the axis thereof. Theliquid crystal molecules 23 continuously form a helical structure in thedirection of extension of the substrates. A half of the apex angle of ahelical cone in this state is a tilt angle H in chiral smectic phase ofsuch a helical structure.

When a voltage higher than a certain threshold level is applied betweenelectrodes formed on the substrates 21a and 21b, a helical structure ofthe liquid crystal molecule 23 is unwound or released to change thealignment direction of respective liquid crystal molecules 23 so thatthe dipole moment (P⊥) 24 ar all directed in the direction of theelectric field. The liquid crystal molecules 23 have an elongated shapeand show refractive anisotropy between the long axis and the short axisthereof. Accordingly, it is easily understood that when, for instance,polarizers arranged in a cross nicol relationship, i.e., with theirpolarizing directions crossing each other, are disposed on the upper andthe lower surfaces of the glass plates, the liquid crystal cell thusarranged functions as a liquid crystal optical modulation device ofwhich optical characteristics vary depending upon the polarity of anapplied voltage.

Further, when the liquid crystal cell is made sufficiently thin (e.g.,0.1-3 microns), the helical structure of the liquid crystal molecules isunwound to provide a non-helical structure even in the absence of anelectric field, whereby the dipole moment assumes either of the twostates, i.e., Pa in an upper direction 34a or Pb in a lower direction34b as shown in FIG. 3, thus providing a bistable condition. When anelectric field Ea or Eb higher than a certain threshold level anddifferent from each other in polarity as shown in FIG. 3 is applied to acell having the above-mentioned characteristics by voltage applicationmeans 31a and 31b, the dipole moment is directed either in the upperdirection 34a or in the lower direction 34b depending on the vector ofthe electric field Ea or Eb. In correspondence with this, the liquidcrystal molecules are oriented in either of a first stable state 33a anda second stable state 33b.

A first advantage attained by using such a ferroelectric liquid crystalcell is that the response speed is quite fast, and a second advantage isthat the orientation of the liquid crystal shows bistability. The secondadvantage will be further explained, e.g., with reference to FIG. 3.When the electric field Ea is applied to the liquid crystal molecules,they are oriented in the first stable state 33a. This state is stablyretained even if the electric field is removed. On the other hand, whenthe electric field Eb of which direction is opposite to that of theelectric field Ea is applied thereto, the liquid crystal molecules areoriented to the second stable state 33b, whereby the directions ofmolecules are changed. This state is similarly stably retained even ifthe electric field is removed. Further, as long as the magnitude of theelectric field Ea or Eb being applied is not above a certain thresholdvalue, the liquid crystal molecules are placed in the respectiveorientation states.

FIG. 4 is a schematic sectional view showing an alignment state ofliquid crystal molecules attained by the present invention, and FIG. 5is a view showing alignment of corresponding C-directors.

Reference numerals 51a and 51b in FIG. 4 denote upper and lowersubstrates, respectively. Numeral 50 denotes a molecular layer composedof liquid crystal molecules 52, and liquid crystal molecules 52 arealigned so as to change their positions along the bottom face 54(circular) of a cone 54. FIG. 5 more specifically shows a change inC-directors. Referring to FIG. 5, at U₁ are shown C-directors 81 (eachbeing a projection of a molecular long axis onto an imaginary planeperpendicular to the normal to a molecular layer 50) in one stableorientation state, and at U₂ are shown C-directors 81 in the otherstable orientation state.

On the other hand, an alignment state attained by a conventionalrubbing-treated polyimide film may be represented by a C-directordiagram of FIG. 6, which shows an alignment state wherein molecular axesare twisted in a large degree from the upper substrate 51a to the lowersubstrate 51b to provide a smaller tilt angle θ.

FIG. 7A is a schematic plan view illustrating a tilt angle θ in analignment state where C-directors 81 assume a state shown in FIG. 5(referred to as "uniform alignment state"), and FIG. 7B is a schematicplan view illustrating a tilt angle θ in an alignment state whereC-directors 81 assume a state shown in FIG. 6 (referred to as "splayalignment state"). In these figures, reference numeral 70 denotes arubbing axis provided to the above-mentioned fluorine-containingpolyimide film, numeral 71a denotes an average molecular axis in theorientation state U₁, numeral 71b denotes an average molecular axis inthe orientation state U₂, numeral 72a denotes an average molecular axisin the orientation state S₁, and numeral 72b denotes an averagemolecular axis in the orientation state S₂. the average molecular axes71a and 71b can be switched to each other by applying voltages ofmutually opposite polarities. Similar switching is caused between theaverage molecular axes 72a and 72b.

Next, the effectiveness of the uniform alignment state with respect to adelay in optical response (after-image) due to a reverse electric fieldVrev is explained.

If the capacitance of an insulating layer constituting a liquid crystalcell is denoted by Ci, the capacitance of a liquid crystal layer isdenoted by C_(LC) and the spontaneous polarization of the liquid crystalis denoted by P_(S), Vrev causing after-image is expressed by thefollowing equation.

    Vrev=2P.sub.S /(Ci+C.sub.LC)

FIG. 8 is a schematic sectional view illustrating changes in chargedistribution direction of P_(S) and direction of the reverse electricfield in a liquid crystal cell. At FIG. 8(a), there is shown adistribution of ⊕ and ⊖ charges in a memory state before application ofa pulse electric field, where the spontaneous polarization is directedfrom ⊕ charges to ⊖ charges. At FIG. 8(b) is shown a state immediatelyafter removal of a pulse electric field, when the direction of thespontaneous polarization P_(S) is opposite to that shown at FIG. 8(a)(thus, the liquid crystal molecules are inverted from one stableorientation state to the other orientation state) but the distributionof the ⊕ and ⊖ charges is similar to that shown at FIG. 8(a), so that areverse electric field Vrev is generated as indicated by a arrow shownat FIG. 8(b). The reverse electric field Vrev disappears in a short timeto provide a distribution of ⊕ and ⊖ charges as shown at FIG. 8(c).

FIG. 9 is a plan view showing a change in optical response in a splayalignment state given by a conventional polyimide alignment film interms of a change in tilt angle θ. Referring to FIG. 9, at the time ofapplication of a pulse electric field, the orientation of liquid crystalmolecules is changed from an average molecular axis S(A) in a splayalignment state to be overshot to an average molecular axis U₂ in auniform alignment state close to that providing a maximum tilt angle Halong a path denoted by an arrow X₁, and immediately after the removalof the pulse electric field, the orientation is changed along a pathdenoted by an arrow X₂ to an average molecular axis S(B) in a splayalignment state providing a decreased tilt angle θ due to the action ofthe reverse electric field Vrev shown at FIG. 8(b). Then, as the reverseelectric field Vrev attenuates as shown at FIG. 8(c), the orientation ischanged along a path denoted by an arrow X₃ to an average molecular axisS(C) in a splay alignment state providing a stable orientation statehaving a somewhat increased tilt angle θ. The resultant optical responsein this case is shown in FIG. 10.

In the alignment state given by using the above-mentioned polyimide filmof the specific structure of the present invention, the averagemolecular axes S(A), S(B) and S(C) in the splay alignment state shown inFIG. 9 are not caused but it is possible to form an alignment state withan average molecular axis giving a tilt angle 0 which is close to amaximum tilt angle H. An optical response at this time according to thepresent invention is shown in FIG. 11. FIG. 11 shows that a delay inoptical response causing after-image is obviated and a high contrast inmemory states is caused.

More specifically, when the alignment film of the present invention isused, it is possible to provide the above-mentioned uniform alignmentstate. This is particularly true when the above-mentioned recurring unitof the formula (I) includes a fluorine atom in the group R₁ or asubstituent X₁ -X₈, which promotes the uniform alignment state toprovide a liquid crystal device showing a high contrast.

Further, when the compounds of, e.g., the above formulae (4)-(8), (12),(15), (21), (35), (40), (42), (43) and (48) are used, it is preferred toapply an AC application treatment after the cell preparation. The ACvoltage used for this purpose may comprise an amplitude of 5-100 volts,preferably 15-50 volts, and a frequency of 10-500 Hz, preferably 10-200Hz. The AC application treatment may be performed for a period on theorder of several seconds to 10 minutes.

As described above, by the ferroelectric liquid crystal device accordingto the present invention comprising a polyimide alignment film of theabove-mentioned specific formula (I) as a layer on the transparentelectrodes contacting the liquid crystal, it is possible to provide ahigh contrast between the bright and dark states, particularly a verylarge display contrast at the time of multiplexing drive, and also ahigh quality display free from ugly after-image.

Hereinbelow, the present invention will be explained based on Examples.

EXAMPLE 1

Two 1.1 mm-thick glass plates each provided with a 1000 Å-thick ITO filmwere respectively coated with a 3.0 wt. % solution of a polyamide acidrepresented by the formula (II) shown below (showing a number-averagemolecular weight (Mn) as measured by GPC of 6×10⁴) in a mixture solventof N-methylpyrrolidone/n-butylcellosolve=2/1 by means of a spinnerrotating at 3000 rpm. ##STR9## After the coating, the film was subjectedto curing under heating at 250° C. for about an hour to form a 450Å-thick film. The coating film was then rubbed in one direction with anylon-planted cloth.

On one of the two glass plates thus treated, 1.5 microns alumina beadswere dispersed, and the other glass plate was superposed thereon so thattheir rubbing axes were parallel to each other and disposed in the samedirection to form a blank cell.

The blank cell was filled with a ferroelectric smectic liquid crystal("CS-1014" (trade name), available from Chisso K.K.) under vacuum and,after sealing, was gradually cooled from isotropic phase at a rate of0.5° C./hour to 30° C., whereby an alignment was effected. The "CS-1014"liquid crystal in the cell showed the following phase transition series.##STR10## Iso.: isotropic phase, Ch.: cholesteric phase,

SmA: smectic A phase.

SmC*: chiral smectic C phase.

The experiment thereafter was performed at 25° C.

The above-prepared liquid crystal cell was sandwiched between a pair of90 degrees-cross nicol polarizers to provide a liquid crystal device andwas supplied with a pulse of 50 μsec and 30 V. Then, the cross nicolpolarizers were set at the extinction position (providing the darkeststate), and the transmittance through the liquid crystal device at thistime was measured by a photo-multiplier. Then, a pulse of 50 μsec and-30 V was applied to the device, and the transmittance (brightest state)at this time was measured in the same manner, whereby the following datawere obtained.

Tilt angle θ=14 degrees, transmittance in the brightest state=40%,transmittance in the darkest state=1.5%, contrast ratio=27:1.

The delay in optical response causing after-image was 0.2 sec or less.

The liquid crystal device was subjected to multiplexing drive fordisplay using driving waveforms shown in FIG. 12, whereby a high-qualitydisplay with a high contrast was attained. Further, after an imagedisplay of a prescribed character image, the whole picture area waserased into "white", whereby no after-image was recognized. Referring toFIG. 12, at S_(N), S_(N+1) and S_(N+2) are shown voltage waveformsapplied to scanning lines, at I is shown a voltage waveform applied to arepresentative date line, and at (I-S_(N)) is shown a combined voltagewaveform applied to the data line I and the scanning line S_(N). In theabove embodiment, the drive was performed under the conditions of V₀=5-8 volts and ΔT=20-70 μsec.

EXAMPLE 2

A liquid crystal cell was prepared in the same manner as in Example 1except that the alignment control films were formed by a polyimide(Mn=5×10⁴) represented by the recurring unit of the following formula(III): ##STR11##

The resultant cell was supplied with an AC voltage of 20 volts and 50 Hzfor 1 minute as a pretreatment, and then subjected to the measurement ofcontrast ratio and optical response in the same manner in Example 1,whereby the contrast was 24:1 and the delay time was 0.4 sec.

EXAMPLES 3-12

Liquid crystal cells were prepared in the same manner as in Example 1except that the alignment control films (in terms of polyimide recurringunits, Mn=4×10⁴ -5×10⁴) and liquid crystal materials shown in Table 1below were used.

The respective cells were tested in the same manner as in Example 1(except that the cells of Examples 4 and 11 were subjected to the sameAC voltage application pretreatment as in Example 2), whereby measureddata of contrast ratio and delay time in optical response shown in Table2 appearing hereinafter were obtained.

The respective cells were subjected to the multiplexing drive fordisplay in the same manner as in Example 1, whereby similar results wereattained with respect to contrast and after-image.

                                      TABLE 1                                     __________________________________________________________________________                                            Liquid crystal                        Ex.                                                                              Alignment control film               material                              __________________________________________________________________________        ##STR12##                           "CS-1014" (trade name) (FLC,                                                  Chisso) k.k.)                         4                                                                                 ##STR13##                           Liquid crystal 3 mentioned-                                                   above                                 5                                                                                 ##STR14##                           Liquid crystal 3 mentioned-                                                   above                                 6                                                                                 ##STR15##                           "CS-1014"                             7                                                                                 ##STR16##                           "CS-1014"                             8                                                                                 ##STR17##                           "                                     9                                                                                 ##STR18##                           "                                     10                                                                                ##STR19##                           Liquid crystal (3)                    11                                                                                ##STR20##                           Liquid crystal (3)                    12                                                                                ##STR21##                           Liquid crystal (3)                    __________________________________________________________________________

                  TABLE 2                                                         ______________________________________                                                      Contrast Delay in optical                                       Example       ratio    response (sec)                                         ______________________________________                                        3             20:1     0.2                                                    4             18:1     0.3                                                    5             22:1     0.3                                                    6             17:1     0.2                                                    7             15:1     0.3                                                    8             20:1     0.3                                                    9             21:1     0.2                                                    10            22:1     0.2                                                    11            21:1     0.3                                                    12            19:1     0.3                                                    ______________________________________                                    

COMPARATIVE EXAMPLES 1-4

Liquid crystal cells were prepared in the same manner as in Example 1except that the alignment control films (in terms of commerciallyavailable precursor polyamide acid varnish and liquid crystal materialsshown in Table 3 below were used. The measured data of contrast ratioand delay in optical response measured for each of the cells are shownin Table 4 below.

The respective cells were subjected to the multiplexing drive fordisplay in the same manner as in Example 1, whereby the resultantcontrasts were smaller than that given by Example 1 and after-image wasrecognized for each cell.

                  TABLE 3                                                         ______________________________________                                        Comparative                                                                            Alignment film                                                       Example  (polyamide acid varnish)                                                                       Liquid crystal material                             ______________________________________                                        1        "SP-710" (trade name)                                                                          "CS-1014"                                                    (aromatic polyimide                                                                            (trade name)                                                 varnish, Toray K.K.)                                                                           (FLC, Chisso K.K.)                                  2        "SP-710" (trade name)                                                                          Liquid crystal material                                      (aromatic polyimide                                                                            (3) described                                                varnish, Toray K.K.)                                                                           hereinbefore                                        3        "LQ-5200" (trade name)                                                                         "CS-1014"                                                    (polyimide varnish,                                                           Hitachi Kasei K.K.)                                                  4        "LQ-5200" (trade name)                                                                         Liquid crystal material                                      (polyimide varnish,                                                                            (3)                                                          Hitachi Kasei K.K.)                                                  ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Comp.                 Delay in                                                Example    Contrast ratio                                                                           optical response (sec)                                  ______________________________________                                        1          6:1        1.5                                                     2          5:1        2.5                                                     3          6:1        1.2                                                     4          5:1        2.2                                                     ______________________________________                                    

As described above, according to the liquid crystal device of thepresent invention, it is possible to provide a high-quality displaywhich is characterized by a high contrast between the bright and darkstates, particularly a very large contrast at the time of multiplexingdrive and yet free from unpleasant after-image.

What is claimed is:
 1. A liquid crystal device, comprising: a pair ofsubstrates and a ferroelectric liquid crystal disposed between thesubstrates; at least one of said pair of substrates having thereon analignment film comprising a polyimide having a recurring unit of thefollowing formula (I): ##STR22## wherein R₁ denotes a tetravalentorganic residue; and X₁ -X₈ independently denote an alkyl group having1-15 carbon atoms, an alkoxy group having 1-15 carbon atoms, CF₃ group,halogen or hydrogen atom, with the proviso that at least one of X₁ -X₄is not hydrogen atom, at least one of X₅ -X₈ is not hydrogen atom, andat least one of X₂, X₄, X₅ and X₇ is not hydrogen atom.
 2. A deviceaccording to claim 1, wherein R₁ in the formula (I) is selected from thegroup consisting of: ##STR23##
 3. A device according to claim 1, whereinR₁ in the formula (I) is selected from the group consisting of:##STR24##
 4. A device according to claim 1, wherein at least one of X₁-X₈ denotes an alkyl or alkoxy group having 1-10 carbon atoms.
 5. Adevice according to claim 1, wherein at least one of X₁ -X₈ denotes analkyl or alkoxy group having 1-5 carbon atoms.
 6. A device according toclaim 1, wherein said ferroelectric liquid crystal assumes a uniformalignment state under the action of the alignment film.
 7. A deviceaccording to claim 6, wherein said alignment film has been subjected torubbing.
 8. A device according to claim 1, wherein said alignment filmhas a thickness of 30 Å-1 μm.
 9. A device according to claim 1, whereinsaid alignment film has a thickness of 200-2000 Å.